Antennas for mobile wireless communications is dictated by a number of factors, but mainly the volume available for the antenna, the frequency (directly related to this volume) of operation and unique environmental constraints of the wireless communication path (also related to frequency of operation), such as the distance over which wireless communication is to be performed, path loss and such like.
Antennas focus radiated RF energy in it radiation pattern such that there appears to be more power coming from the antenna in a particular direction. The electrical characteristics of an antenna, such as gain, radiation pattern, impedance, bandwidth, resonant frequency and polarization, are the same whether the antenna is transmitting or receiving.
The term antenna gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Gain is a key performance figure which combines the antenna's directivity and electrical efficiency. Antenna gain is usually defined as the ratio of the power produced by the antenna from a far-field source on the antenna's beam axis to the power produced by a hypothetical lossless isotropic antenna, which is equally sensitive to signals from all directions. Usually this ratio is expressed in decibels, and these units are referred to as “decibels-isotropic” (dBi). An alternate definition compares the antenna to the power received by a lossless half-wave dipole antenna, in which case the units are written as dBd.
Antenna gain is sometimes referred to as a function of angle, but when a single number is quoted the gain is the ‘peak gain’ over all directions.
Directivity measures how much more intensely the antenna radiates in its preferred direction than a mythical “isotropic radiator” when fed with the same total power. It follows then that the higher the gain of an antenna the smaller the effective angle of use. This directly impacts the choice of the antenna for a specific function. To achieve a directivity which is significantly greater than unity, the antenna size needs to be much larger than the wavelength. This can usually achieved using a phased array of half-wave or full-wave antennas. Since a phased array is comprised of a number of individual physically separate antennas, a phased array is not an adequate solution for particular mobile wireless communications due to the size of the aggregated individual antennas plus the gap distance between them.
An antenna radiation pattern is a graphical representation of the intensity of the radiation versus the angle from a perpendicular to a plane of the antenna. The graph is usually circular, the intensity indicated by the distance from the centre based in the corresponding angle. The radiation pattern may be used to determine the beamwidth which is generally accepted as the angle between the two points (on the same plane) at which the radiation falls to “half power” i.e. 3 dB below the point of maximum radiation.
Antenna impedance relates the voltage to the current at the input to the antenna. The real part of the antenna impedance represents power that is either radiated away or absorbed within the antenna. The imaginary part of the impedance represents power that is stored in the near field of the antenna. This is non-radiated power. An antenna with only a real part input impedance (zero imaginary part) is said to be resonant. Note that the impedance of an antenna will vary with frequency. A common measure of how well matched the antenna is to the feed line (transmission line) or receiver is known as the Voltage Standing Wave Ratio (VSWR). VSWR is a real number that is always greater than or equal to 1. A VSWR of 1 indicates no mismatch loss (the antenna is perfectly matched to the transmission line). Higher values of VSWR indicate more mismatch loss.
Although a resonant antenna has by definition an almost purely resistive feed-point impedance at a particular frequency, many (if not most) applications require using an antenna over a range of frequencies. An antenna's bandwidth specifies the range of frequencies over which its performance does not suffer due to a poor impedance match. Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, a return loss S11=20*log 10(0.2)=−13.98 dB. Note that the above does not imply that 96% of the power delivered to the antenna is transmitted in the form of electromagnetic radiation; losses must still be taken into account.
Antenna conductors have the lowest feed-point impedance at the resonant frequency where they are just under ¼ wavelength long. The reason a dipole antenna is used at the resonant frequency is not that the ability of a resonant antenna to transmit (or receive) fails at frequencies far from the resonant frequency but has to do with the impedance match between the antenna and the transmitter or receiver (and its transmission line).
As mentioned earlier, higher the gain of an antenna the smaller the effective angle of use. This directly impacts the choice of the antenna for a specific function. In mobile cellular applications the factors discussed above play an important consideration in trying to realize a small form factor efficient antenna.
In recent years, there is increasing interest in the worldwide unlicensed band at 60 GHz for wireless data communication services. The frequency bands for the operation of these millimeter wavelength radio frequencies are different for various regions in the world. In 2001, the United States Federal Communications Commission (FCC) released 7 GHz of bandwidth (57-64) GHz for unlicensed use, while other governments have similarly allowed portions of the 60 GHz band to be used without a license. The use of this frequency band offers interesting features such as high available bandwidth and high capacity for mobile data communication services. However, the main disadvantages of the unlicensed band at 60 GHz are the high path losses (30 dB higher than 2 GHZ) and the oxygen absorption (around 10-15 dB/km). Patches and dielectric resonator antennas may be used to provide high gain at 60 GHz however these have a disadvantage of having to tradeoff one or more characteristics such as gain, directivity and bandwidth.